1. Field of the Invention
This invention relates to removing and recovering methane and higher boiling hydrocarbons from a natural gas stream which contains large quantities of inert gases, may contain acidic components such as CO.sub.2 and H.sub.2 S, and may vary in moisture content from dry to saturated. It specifically relates to the upgrading of heating values of natural gas streams having heating values below desired specifications. It further relates to adapting the extractive flashing and extractive stripping versions of the Mehra Process for processing of nitrogen-rich natural gas streams.
2. Review of the Prior Art
Many hydrocarbon gases, such as natural gas, are contaminated with one or more inert gases which lower their heat content or otherwise impair their marketability. Such inert gases include nitrogen, helium, and argon.
During recent years, there has been strong emphasis on the secondary and tertiary methods of recovering oil from formations where the primary oil-producing methods are no longer productive. Nitrogen injection for reviving these oil wells is not useful in most formations, but in some formations such as in the central and north Texas areas of the United States, nitrogen injection has been successfully utilized for the recovery of additional oil.
After several years of nitrogen injections at high pressure, approximately 14,000 Kpa (2,030 psia), the nitrogen seems to have broken through the formations in many instances. In other words, nitrogen is coming out with the oil, and it is separated from the oil at the separator. Previously, the associated gases were rich in hydrocarbons heavier than methane, along with substantial quantities of methane. The present dilution effect of nitrogen has caused the same associated wellhead gas to have an extremely low heat content, thereby making it unsuitable for pipeline shipments. If the natural gas contains more than 3% of nitrogen, it is off-specification for most of the world's pipelines.
This situation has caused the oil producer to curtail oil production because government regulations prevent him from burning the nitrogen-rich associated gas, and both environmental laws and a desire to preserve valuable resources prohibit him from venting the associated hydrocarbons. The oil producer is thus limited by the choice of technology available to him for properly processing the associated gases from an oil well. The prior art technology, which involves cryogenic principles, cannot economically process the natural gas streams which contain more than 3% nitrogen even after subsidization with the revenues from oil production.
Natural gas is a mixture of hydrocarbons, including methane, ethane, propane, and various amounts of higher molecular weight hydrocarbons together with nitrogen and acid gases, such as CO.sub.2 and/or H.sub.2 S. A "dry" gas is one containing predominantly methane with some ethane, propane, and butane and having a very low hydrocarbon dew point. The heavier the hydrocarbons, such as pentane and higher homologs, that are present in the gas, the higher the hydrocarbon dew point. For pipeline transmission, enough of the heavier hydrocarbons must be removed to lower the dew point without losing too many of the calories needed to meet specifications. In the past, gases with large quantities of high molecular weight hydrocarbons have been passed through gasoline extraction plants and/or dew point control stations to lower the dew point. Also, frequently the gas has required conditioning to remove sulfur compounds and carbon dioxide.
Inability to change the composition of this liquid in accordance with market conditions has often been a handicap. The extractive flashing version of the Mehra Process, as described in U.S. Pat. Nos. 4,421,535, 4,511,381, 4,526,594, and 4,578,094, and the extractive stripping version of the Mehra Process, as described in U.S. Pat. Nos. 4,617,038, 4,692,179 have provided a solution for this problem with respect to recovering C.sub.2 + hydrocarbons.
In addition, an improved extractive flashing version and an improved extractive stripping version of the Mehra Process are respectively described in U.S. Pat. Nos. 4,623,371 and 4,680,042 for separating C.sub.2 + hydrocarbons from a nitrogen-rich hydrocarbon gas containing from 3 to 75 mol % nitrogen, the remainder being hydrocarbons. U.S. Pat. Nos. 4,623,371 and 4,680,042 are incorporated herein by reference.
These patents disclose processes capable of removing an inert gas from an inert-rich natural gas stream by oil producers who have been using nitrogen injection and by natural gas producers whose wells contain nitrogen, helium, and/or argon. In addition, these processes produce an acceptable hydrocarbon gas product and a natural gas liquids product from an inert-rich hydrocarbon gas stream and selectively adjust the heat content of the gas product and the hydrocarbon contents of the liquids product in accordance with market economics, thereby enablng the operator of a natural gas liquids extraction plant to minimize ethane recovery and maximize propane recovery, for example, in response to market conditions, while also producing an N.sub.2 -rich gas product.
In areas where the natural gas contains more than 3 mol % of nitrogen, helium, and/or other inert gases, there are many absorber plants utilizing lean oils as the solvent for recovering C.sub.2 + hydrocarbons as the natural gas liquids (NGL) product. Because of the nitrogen content of the residue gas, it is often out of limits as a sales gas. These plants may be operating under borderline economic conditions and for economic reasons need a method for continuing operation while utilizing the same solvent. Lean oils which are customarily circulated in such absorber plants are low in selectivity capability for hydrocarbons, generally having a relative volatility of methane over ethane that is less than 5.0 and a hydrocarbon loading capacity, defined as solubility of ethane in the lean oils, of less than 3.5-34 normal cubic meters of ethane per cubic meter (0.49-4.8 standard cubic feet of ethane per gallon) of solvent.